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rfc:rfc1995

Network Working Group M. Ohta Request for Comments: 1995 Tokyo Institute of Technology Updates: 1035 August 1996 Category: Standards Track

                  Incremental Zone Transfer in DNS

Status of this Memo

 This document specifies an Internet standards track protocol for the
 Internet community, and requests discussion and suggestions for
 improvements.  Please refer to the current edition of the "Internet
 Official Protocol Standards" (STD 1) for the standardization state
 and status of this protocol.  Distribution of this memo is unlimited.

Abstract

 This document proposes extensions to the DNS protocols to provide an
 incremental zone transfer (IXFR) mechanism.

1. Introduction

 For rapid propagation of changes to a DNS database [STD13], it is
 necessary to reduce latency by actively notifying servers of the
 change.  This is accomplished by the NOTIFY extension of the DNS
 [NOTIFY].
 The current full zone transfer mechanism (AXFR) is not an efficient
 means to propagate changes to a small part of a zone, as it transfers
 the entire zone file.
 Incremental transfer (IXFR) as proposed is a more efficient
 mechanism, as it transfers only the changed portion(s) of a zone.
 In this document, a secondary name server which requests IXFR is
 called an IXFR client and a primary or secondary name server which
 responds to the request is called an IXFR server.

2. Brief Description of the Protocol

 If an IXFR client, which likely has an older version of a zone,
 thinks it needs new information about the zone (typically through SOA
 refresh timeout or the NOTIFY mechanism), it sends an IXFR message
 containing the SOA serial number of its, presumably outdated, copy of
 the zone.

Ohta Standards Track [Page 1] RFC 1995 Incremental Zone Transfer in DNS August 1996

 An IXFR server should keep record of the newest version of the zone
 and the differences between that copy and several older versions.
 When an IXFR request with an older version number is received, the
 IXFR server needs to send only the differences required to make that
 version current.  Alternatively, the server may choose to transfer
 the entire zone just as in a normal full zone transfer.
 When a zone has been updated, it should be saved in stable storage
 before the new version is used to respond to IXFR (or AXFR) queries.
 Otherwise, if the server crashes, data which is no longer available
 may have been distributed to secondary servers, which can cause
 persistent database inconsistencies.
 If an IXFR query with the same or newer version number than that of
 the server is received, it is replied to with a single SOA record of
 the server's current version, just as in AXFR.
 Transport of a query may be by either UDP or TCP.  If an IXFR query
 is via UDP, the IXFR server may attempt to reply using UDP if the
 entire response can be contained in a single DNS packet.  If the UDP
 reply does not fit, the query is responded to with a single SOA
 record of the server's current version to inform the client that a
 TCP query should be initiated.
 Thus, a client should first make an IXFR query using UDP.  If the
 query type is not recognized by the server, an AXFR (preceded by a
 UDP SOA query) should be tried, ensuring backward compatibility.  If
 the query response is a single packet with the entire new zone, or if
 the server does not have a newer version than the client, everything
 is done.  Otherwise, a TCP IXFR query should be tried.
 To ensure integrity, servers should use UDP checksums for all UDP
 responses.  A cautious client which receives a UDP packet with a
 checksum value of zero should ignore the result and try a TCP IXFR
 instead.
 The query type value of IXFR assigned by IANA is 251.

3. Query Format

 The IXFR query packet format is the same as that of a normal DNS
 query, but with the query type being IXFR and the authority section
 containing the SOA record of client's version of the zone.

Ohta Standards Track [Page 2] RFC 1995 Incremental Zone Transfer in DNS August 1996

4. Response Format

 If incremental zone transfer is not available, the entire zone is
 returned.  The first and the last RR of the response is the SOA
 record of the zone.  I.e. the behavior is the same as an AXFR
 response except the query type is IXFR.
 If incremental zone transfer is available, one or more difference
 sequences is returned.  The list of difference sequences is preceded
 and followed by a copy of the server's current version of the SOA.
 Each difference sequence represents one update to the zone (one SOA
 serial change) consisting of deleted RRs and added RRs.  The first RR
 of the deleted RRs is the older SOA RR and the first RR of the added
 RRs is the newer SOA RR.
 Modification of an RR is performed first by removing the original RR
 and then adding the modified one.
 The sequences of differential information are ordered oldest first
 newest last.  Thus, the differential sequences are the history of
 changes made since the version known by the IXFR client up to the
 server's current version.
 RRs in the incremental transfer messages may be partial.  That is, if
 a single RR of multiple RRs of the same RR type changes, only the
 changed RR is transferred.
 An IXFR client, should only replace an older version with a newer
 version after all the differences have been successfully processed.
 An incremental response is different from that of a non-incremental
 response in that it begins with two SOA RRs, the server's current SOA
 followed by the SOA of the client's version which is about to be
 replaced.
 5. Purging Strategy
 An IXFR server can not be required to hold all previous versions
 forever and may delete them anytime. In general, there is a trade-off
 between the size of storage space and the possibility of using IXFR.
 Information about older versions should be purged if the total length
 of an IXFR response would be longer than that of an AXFR response.
 Given that the purpose of IXFR is to reduce AXFR overhead, this
 strategy is quite reasonable.  The strategy assures that the amount
 of storage required is at most twice that of the current zone
 information.

Ohta Standards Track [Page 3] RFC 1995 Incremental Zone Transfer in DNS August 1996

 Information older than the SOA expire period may also be purged.

6. Optional Condensation of Multiple Versions

 An IXFR server may optionally condense multiple difference sequences
 into a single difference sequence, thus, dropping information on
 intermediate versions.
 This may be beneficial if a lot of versions, not all of which are
 useful, are generated. For example, if multiple ftp servers share a
 single DNS name and the IP address associated with the name is
 changed once a minute to balance load between the ftp servers, it is
 not so important to keep track of all the history of changes.
 But, this feature may not be so useful if an IXFR client has access
 to two IXFR servers: A and B, with inconsistent condensation results.
 The current version of the IXFR client, received from server A, may
 be unknown to server B. In such a case, server B can not provide
 incremental data from the unknown version and a full zone transfer is
 necessary.
 Condensation is completely optional. Clients can't detect from the
 response whether the server has condensed the reply or not.
 For interoperability, IXFR servers, including those without the
 condensation feature, should not flag an error even if it receives a
 client's IXFR request with a unknown version number and should,
 instead, attempt to perform a full zone transfer.

7. Example

 Given the following three generations of data with the current serial
 number of 3,
    JAIN.AD.JP.         IN SOA NS.JAIN.AD.JP. mohta.jain.ad.jp. (
                                      1 600 600 3600000 604800)
                        IN NS  NS.JAIN.AD.JP.
    NS.JAIN.AD.JP.      IN A   133.69.136.1
    NEZU.JAIN.AD.JP.    IN A   133.69.136.5
 NEZU.JAIN.AD.JP. is removed and JAIN-BB.JAIN.AD.JP. is added.
    jain.ad.jp.         IN SOA ns.jain.ad.jp. mohta.jain.ad.jp. (
                                      2 600 600 3600000 604800)
                        IN NS  NS.JAIN.AD.JP.
    NS.JAIN.AD.JP.      IN A   133.69.136.1
    JAIN-BB.JAIN.AD.JP. IN A   133.69.136.4
                        IN A   192.41.197.2

Ohta Standards Track [Page 4] RFC 1995 Incremental Zone Transfer in DNS August 1996

 One of the IP addresses of JAIN-BB.JAIN.AD.JP. is changed.
    JAIN.AD.JP.         IN SOA ns.jain.ad.jp. mohta.jain.ad.jp. (
                                      3 600 600 3600000 604800)
                        IN NS  NS.JAIN.AD.JP.
    NS.JAIN.AD.JP.      IN A   133.69.136.1
    JAIN-BB.JAIN.AD.JP. IN A   133.69.136.3
                        IN A   192.41.197.2
 The following IXFR query
               +---------------------------------------------------+
    Header     | OPCODE=SQUERY                                     |
               +---------------------------------------------------+
    Question   | QNAME=JAIN.AD.JP., QCLASS=IN, QTYPE=IXFR          |
               +---------------------------------------------------+
    Answer     | <empty>                                           |
               +---------------------------------------------------+
    Authority  | JAIN.AD.JP.         IN SOA serial=1               |
               +---------------------------------------------------+
    Additional | <empty>                                           |
               +---------------------------------------------------+
 could be replied to with the following full zone transfer message:
               +---------------------------------------------------+
    Header     | OPCODE=SQUERY, RESPONSE                           |
               +---------------------------------------------------+
    Question   | QNAME=JAIN.AD.JP., QCLASS=IN, QTYPE=IXFR          |
               +---------------------------------------------------+
    Answer     | JAIN.AD.JP.         IN SOA serial=3               |
               | JAIN.AD.JP.         IN NS  NS.JAIN.AD.JP.         |
               | NS.JAIN.AD.JP.      IN A   133.69.136.1           |
               | JAIN-BB.JAIN.AD.JP. IN A   133.69.136.3           |
               | JAIN-BB.JAIN.AD.JP. IN A   192.41.197.2           |
               | JAIN.AD.JP.         IN SOA serial=3               |
               +---------------------------------------------------+
    Authority  | <empty>                                           |
               +---------------------------------------------------+
    Additional | <empty>                                           |
               +---------------------------------------------------+

Ohta Standards Track [Page 5] RFC 1995 Incremental Zone Transfer in DNS August 1996

 or with the following incremental message:
               +---------------------------------------------------+
    Header     | OPCODE=SQUERY, RESPONSE                           |
               +---------------------------------------------------+
    Question   | QNAME=JAIN.AD.JP., QCLASS=IN, QTYPE=IXFR          |
               +---------------------------------------------------+
    Answer     | JAIN.AD.JP.         IN SOA serial=3               |
               | JAIN.AD.JP.         IN SOA serial=1               |
               | NEZU.JAIN.AD.JP.    IN A   133.69.136.5           |
               | JAIN.AD.JP.         IN SOA serial=2               |
               | JAIN-BB.JAIN.AD.JP. IN A   133.69.136.4           |
               | JAIN-BB.JAIN.AD.JP. IN A   192.41.197.2           |
               | JAIN.AD.JP.         IN SOA serial=2               |
               | JAIN-BB.JAIN.AD.JP. IN A   133.69.136.4           |
               | JAIN.AD.JP.         IN SOA serial=3               |
               | JAIN-BB.JAIN.AD.JP. IN A   133.69.136.3           |
               | JAIN.AD.JP.         IN SOA serial=3               |
               +---------------------------------------------------+
    Authority  | <empty>                                           |
               +---------------------------------------------------+
    Additional | <empty>                                           |
               +---------------------------------------------------+
 or with the following condensed incremental message:
               +---------------------------------------------------+
    Header     | OPCODE=SQUERY, RESPONSE                           |
               +---------------------------------------------------+
    Question   | QNAME=JAIN.AD.JP., QCLASS=IN, QTYPE=IXFR          |
               +---------------------------------------------------+
    Answer     | JAIN.AD.JP.         IN SOA serial=3               |
               | JAIN.AD.JP.         IN SOA serial=1               |
               | NEZU.JAIN.AD.JP.    IN A   133.69.136.5           |
               | JAIN.AD.JP.         IN SOA serial=3               |
               | JAIN-BB.JAIN.AD.JP. IN A   133.69.136.3           |
               | JAIN-BB.JAIN.AD.JP. IN A   192.41.197.2           |
               | JAIN.AD.JP.         IN SOA serial=3               |
               +---------------------------------------------------+
    Authority  | <empty>                                           |
               +---------------------------------------------------+
    Additional | <empty>                                           |
               +---------------------------------------------------+

Ohta Standards Track [Page 6] RFC 1995 Incremental Zone Transfer in DNS August 1996

 or, if UDP packet overflow occurs, with the following message:
               +---------------------------------------------------+
    Header     | OPCODE=SQUERY, RESPONSE                           |
               +---------------------------------------------------+
    Question   | QNAME=JAIN.AD.JP., QCLASS=IN, QTYPE=IXFR          |
               +---------------------------------------------------+
    Answer     | JAIN.AD.JP.         IN SOA serial=3               |
               +---------------------------------------------------+
    Authority  | <empty>                                           |
               +---------------------------------------------------+
    Additional | <empty>                                           |
               +---------------------------------------------------+

8. Acknowledgements

 The original idea of IXFR was conceived by Anant Kumar, Steve Hotz
 and Jon Postel.
 For the refinement of the protocol and documentation, many people
 have contributed including, but not limited to, Anant Kumar, Robert
 Austein, Paul Vixie, Randy Bush, Mark Andrews, Robert Elz and the
 members of the IETF DNSIND working group.

9. References

 [NOTIFY] Vixie, P., "DNS NOTIFY: A Mechanism for Prompt
 Notification of Zone Changes", RFC 1996, August 1996.
 [STD13] Mockapetris, P., "Domain Name System", STD 13, RFC 1034 and
 RFC 1035), November 1987.

10. Security Considerations

 Though DNS is related to several security problems, no attempt is
 made to fix them in this document.
 This document is believed to introduce no additional security
 problems to the current DNS protocol.

Ohta Standards Track [Page 7] RFC 1995 Incremental Zone Transfer in DNS August 1996

11. Author's Address

 Masataka Ohta
 Computer Center
 Tokyo Institute of Technology
 2-12-1, O-okayama, Meguro-ku, Tokyo 152, JAPAN
 Phone: +81-3-5734-3299
 Fax: +81-3-5734-3415
 EMail: mohta@necom830.hpcl.titech.ac.jp

Ohta Standards Track [Page 8]

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